The present invention relates to coal gasification and more particularly to an improved method and system for handling lock gas generated during operation of a coal gasifier.
In the fixed bed coal gasification process, coal is loaded or charged into the gasifier vessel at its upper end. Each new charge lands on previously-deposited charges. Steam and an oxidizing gas are admitted at the lower end of the gasifier vessel. Combustion occurring near the bottom of the gasifier vessel consumes some of the coal while the rest of the coal is gasified to yield a raw gas.
In one type of fixed bed coal gasification process, a relatively constant superatmospheric pressure is maintained in the gasifier vessel. In a pressurized gasifier system, coal may be introduced into the gasifier by means of a charging lock or lockhopper arrangement. A quantity or charge of coal is loaded into the lockhopper at atmospheric pressure while the lockhopper is sealed from the gasifier vessel. After the lockhopper is fully loaded, the lockhopper internal pressure is elevated by a high pressure filling gas until it is on the order of or slightly greater than the gasifier vessel interior pressure. A coal-carrying filling tube between the lower end of the lockhopper and the upper end of the gasifier vessel is then opened to allow the coal to be charged into the vessel.
As the coal is introduced into the gasifier vessel through the filling tube, gas generated within the vessel rises through the filling tube into the lockhopper. This gas is generally referred to as lock gas although its chemical composition is substantially the same as the raw gas which the gasifier vessel is intended to produce.
Since the lockhopper has to be at atmospheric pressure before coal can be loaded into it, the accumulated lock gas must be disposed of before the next loading operation. The simplest way to dispose of lock gas is to exhaust it into the atmosphere as the lockhopper is being depressurized. This method, while simple, is unacceptable for at least two reasons. First, the lock gas contains elements such as sulphur compounds which would contribute to atmospheric pollution if exhausted without treatment. Second, since the lock gas has substantially the same composition as any gas generated within the gasifier vessel, simply exhausting it into atmosphere is a complete waste of its energy potential.
It has been suggested that lock gas might be used as an incinerator fuel. However, there is concern that burning untreated lock gas might contribute to atmospheric pollution. Moreover, the pressure of lock gas exhausted from the lockhopper during depressurization fluctuates widely and nonuniformly. Additionally, each gasifier has its own gas generating characteristics. This lack of uniformity among gasifiers, which may have a common lock gas handling system, and the widely fluctuating lock gas pressure from any one gasifier further tend to discourage use of lock gas as an incinerator fuel.
It has also been suggested that expanded or low pressure lock gas might be re-compressed and applied to gas cleanup equipment along with raw gas generated in the gasifier vessel. However, since the pressure of expanded lock gas is on the order of atmospheric pressure or 14.7 pounds per square inch while the raw gas pressure is on the order of 330 pounds per square inch, a considerable amount of energy must be expended in re-compressing the lock gas. This reduces the overall efficiency of the coal gasification process.